1. Field of the Invention
The present invention relates to en electronic system by which a motor (e.g.--a permanent magnet d.c. servo motor) is caused to rotate from a first angular position, .theta..sub.1, to a second angular position, O.sub.2, at an angular speed .omega. dependent upon the input signals to the system.
2. Description of Prior Art
Digital positioning systems are typically characterized by how digital data, corresponding to the "move distance," is loaded into the system.
In parallel input systems, a parallel binary or BCD digital word having a plurality of bits corresponding to the distance to be moved is simultaneously loaded into the system. In a typical system the parallel word is preset into a down counter 100 (FIG. 1). The parallel outputs of the down counter 100 are connected to a digital to analog (D/A) converter 101 which ultimately drives the servo amplifier used to drive the motor. As the motor begins to rotate, pulses are emitted from an incremental encoder mechanically connected to the motor shaft. N pulses per revolution are emitted, where N is the encoder line density. The encoder pulses cause the count in the down counter 100 to decrement by one count for each encoder pulse. It can be seen that the contents of the down counter 100 represents the distance yet to be moved and, since this information (via the D/A converter 100) is ultimately used to drive the motor, when the contents of the down counter 100 reaches a zero count, the drive signal from the servo amplifier to the motor will be zero and the motor's motion will cease.
Since move distance data is loaded in parallel, there is no inherent external means by which speed may be synchronously (zero long term speed error) controlled during a move. This is a serious drawback in single or multiple axis systems where speed as well as position must be known and controlled on a real time basis (e.g.--a two axis contouring system).
Parallel input systems, then, are characterized by the ability to perform point to point positioning functions at some preset speed, usually internally programmed. Speed is not easily synchronized to an external source during a move.
Serial input systems, on the other hand, load data serially, one bit at a time (FIG. 2), into an up/down counter 102. Under normal operating conditions, input pulses cause the counter 102 to count up, encoder pulses cause it to count down. As in the parallel system, the parallel outputs of the up/down counter drive a D/A converter 103 which drives the servo amplifier. In a serial input system, however, the rate at which serial input pulses are loaded corresponds exactly to motor speed (proportional to the rate at which encoder pulses count the counter down) and the total quantity of input pulses corresponds exactly to the distance to be moved (total number of encoder pulses). Consequently serial input systems are capable of moving a motor shaft through a precise angle and can simultaneously precisely control the motor's angular velocity during a move. Serial input systems therefore perform a dual role of enabling both precise velocity control and precise position control, making them more useful for those applications requiring these features.